1. Field of the Invention.
This invention relates to fiber optic transmission lines and more particularly to devices for coupling to optical fibers for separating a predetermined portion of the multiplexed light signal transmitted therein and to respond discretely to the demultiplexed signals.
2. Description of the Prior Art.
The field of fiber optics has progressed in a relatively few years from laboratory curiosities and decorative pieces to present-day systems of high sophistication for optical communication and data transmission. Optical fibers or light tubes are specially fabricated filaments which exhibit the property of transmitting light longitudinally along a flexible axis. Various materials which can be used in fabrication of optical fibers and the particular properties thereof are described in Derick et al U.S. Pat. No. 3,508,589 and, in further detail, in British Pat. No. 1,037,498, cited therein. Low-loss fiber optic taps are important components for fiber optic data links and data buses. This is true because it is desirable to be able to tap a portion of a signal propagating through an optical fiber without breaking or terminating the fiber, since fiber terminations add unwanted optical losses to the system and unfavorably increase the need for highly precise fiber splicing and interconnecting arrangements. Since fiber optic transmission lines having a large number of signal taps are inherently power-starved, it is important to minimize excess losses associated with these components. Furthermore, it is desirable to have taps which can be fabricated so that the tap ratio (the power output of the tap divided by the power into the fiber in a given direction) can be conveniently tailored to the unique requirements of the given system. Efficient fiber optic taps have been reported previously whereby two fibers are cleaved, or ground and polished, at specific angles and butt joined. For example, see Karr et al, "Lightwave Fiber Tap", Applied Optics, Vol. 17, page 2215 (July 15, 1978) and Kuwahara et al, "A Semi-Transparent Mirror-Type Directional Coupler for Optical Fiber Applications", IEEE Transactions on Microwave Theory and Technique, Vol. 23, page 179 (Jan. 1975). In these examples, the tap ratio is variable either by changing the cleavage angle or by using materials with different indices of refraction between the cleaved surfaces. However, devices fabricated by such methods are quite fragile and cannot be easily reproduced with sufficient accuracy.
It has been demonstrated that when an optical fiber is bent in the form of an arc, there is an increased tendency for light to escape from the bent region in a radiation pattern which is primarily in the plane of the bend and which is directed away from the center of curvature. See, for example, Gambling et al, "Radiation From Curved Single-Mode Fibers", Electronics Letters, Vol. 12, page 567 (Oct. 14, 1976); and Goell et al U.S. Pat. No. 3,982,123. The tendency for light to escape from the bent region of the fibers is enhanced when a flat region is lapped and polished into the fiber surface perpendicular to the radius of the bend in the fiber.
The advantages of multiplexing such fiber optic systems are obvious. That is, with either frequency or wavelength multiplexing of a fiber optic signal, a plurality of separate and distinct signals, each carrying different data, may utilize a single optical fiber. However, in order to do this, the output of the multiplexed signal must be divided so that each channel can be separately utilized. Many different structures have been suggested for performing the multiplexing and demultiplexing steps. For instance, U.S. Pat. No. 4,001,577 of Albanese et al attaches plural separate fibers to a waveguide with a specific chirp characteristic such that the signals transmitted from the fibers to the waveguide are discriminated in accordance with the chirp and only those of a matched frequency are transmitted through the waveguide to a prism. U.S. Pat. No. 4,054,389 of Owen uses a fiber optic bundle, separates specific fibers in the bundle and attaches them to a wedge-shaped wide transmitting unit. The wedge effectively demultiplexes the signals due to variations in travel distance between the input side of the wedge and the output side containing photodiodes. A diffraction grating system has been suggested in U.S. Pat. Nos. 4,111,524 and 4,153,330 of Tomlinson, III. In these patents plural fibers are positioned at specific angles to each other at the input side of a waveguide which has an integrally formed lens. The light thus passes out of the fibers into the waveguide, through the lens, and is then selectively reflected off of the diffraction grating to demultiplex the signal.
Additionally, numerous systems requiring travel of the light through the atmosphere for interaction with a diffraction grating are suggested in the art. In U.S. Pat. No. 4,146,783 of Desbois et al plural diffraction gratings are used to separate and recombine light during a modification procedure. U.S. Pat. No. 3,863,063 of Indig et al uses a concave grating to spread the multiplexed signal out in the atmosphere and then collimates the spread signal and detects its components. Also, d'Auria et al U.S. Pat. No. 3,953,727 utilizes the spreading cone of light exiting a fiber, collimates the light with a lens and directs the collimated light to a diffraction grating. The grating then splits the light into its components and a lens is used to focus the components on separate diodes.
Since the travel of the light through the atmosphere results in relatively bulky equipment and can result in alignment problems and interference, especially during a multiplexing step, it would be appropriate to find a system which does not depend upon atmospheric travel of the light. Boyd et al suggest such a system in an article entitled "Composite Prism-Grating Coupler For Coupling Light Into High Refractive Index Thin-Film Wave Guides", Applied Optics, Vol. 15, No. 7, page 1681 (July 1976). The article suggests the use of a prism/waveguide interface having a diffraction grating positioned at the interface in order to phase match signals coupled into the waveguide.